Composition and Structure of Zircon from Harzburgite Inclusion Hosted in Granulites of the Bug Complex, Ukrainian Shield

Resumo

The comprehensive study (electron microscopy, mass-spectrometry of secondary ions, Raman spectroscopy) of zircon from harzburgite inclusion in granulites of the Bug complex (Ukrainian Shield) is carried out. Based on a number of features, the studied zircons are subdivided into three groups. Zircon from the first group is of igneous genesis and crystallized 2.8 Ga ago at the temperature of about 650°C from a melt of orthopyroxenite composition. Zircon from the second group underwent hydrothermal alteration ca. 2.75 Ga ago at the temperature of about 750°C. Zircon from the third group is distinctive in several geochemical characteristics (higher Lu/Hf ratio, etc.) and formed before the hydrothermal process, but their compositions were altered by the fluid. Conditions of the igneous zircon (first group) crystallization were characterized by the higher \({{f}_{{{{{\text{O}}}_{{\text{2}}}}}}}\) value, compared to the later hydrothermal alteration. The hydrothermal process has provided the distortion of zircon crystal lattice, which is reflected in the Raman spectra, and the formation of amorphous structure in the uranium-rich zircon from the third group. In terms of trace element contents, the first and second zircon groups indicate different initial melts (from which they originated) from the third zircon group; the melt of the third group was more mafic. The obtained results support the idea about the hybrid character of the melt and that the rock formed in two main stages, namely, a magmatic one and fluid alteration, separated in time and having different oxidation-reduction conditions.